This invention relates to the technical domain of sensors sensitive to the direction and intensity of a magnetic induction to detect the position and/or speed of a mobile target in the general sense.
The purpose of the invention relates particularly to the domain of TPOS (True Power On Sensor) type magnetic sensors capable of supplying information about the position of a mobile target immediately after they are switched on.
Several sensors of this type are known in the state of the art. Thus a position and/or speed sensor is known comprising a mobile target made from a soft material and with at least one and in general a series of mobile teeth in a direction perpendicular to an OZ direction. This type of sensor also comprises a permanent magnet magnetized in the positive or negative direction along the OZ defining an air gap with the mobile target. A probe sensitive to the direction and intensity of a magnetic induction is placed in the air gap such that its axis of greatest sensitivity is approximately along the OZ axis. Conventionally, this type of probe sensitive to the direction and intensity of a magnetic induction is a Hall effect cell.
Displacement of the mobile target causes a variation of the magnetic induction passing through the probe each time that a tooth passes in front of the probe, thus outputting an electrical signal that depends on the direction and the amplitude of the magnetic induction. This sensitive probe is associated particularly with a hysterisis level comparator with an output that is equal to a first logical state when the electrical signal output by the probe is greater than a predetermined threshold, and a second logical state when the electrical signal is less than a predetermined threshold.
Determination of the position of a mobile target requires that an attempt should be made to make the sensor operate at magnetic induction values close to Zero Gauss, while supplying a sufficient magnetic induction excursion to be detectable, in order to be independent of magnet temperature drifts.
To achieve this objective, proposals have been made in the state of the art to make the magnet in the shape of a U, in which the two branches are directed towards the target and the sensitive probe is inserted between the branches to measure the Z component of the magnetic induction. The magnet is provided with a cavity in order to detect the variation of the magnetic induction around an induction value close to Zero Gauss.
The previous known solution has a major disadvantage related to the complex shapes of the magnet that impose the use of composite materials such as rare earth based plasto magnets that are well adapted to making complex shaped magnets by casting, but for which the intrinsic magnetic performances are very much lower than the performances of pure rare earth magnets. Thus, the use of a material with low magnetic performances can have negative effects on the volume of the magnet and the dimension and cost of such a sensor. Furthermore, optimization of the performances of such a sensor depends mainly on the performances of the magnet, and becomes expensive and difficult.
Similarly, patent U.S. Pat. No. 5,781,005 describes a magnet operating at magnetic induction values close to Zero Gauss. This type of sensor comprises a magnetic structure placed behind the plane of a Hall effect probe and placed such that a South pole and a North pole are adjacent to the Hall effect probe. Apart from the difficulties in assembling the probe with respect to the magnetic structure, it should be noted that the magnetic structure is set back from the measurement air gap such that the magnetic induction is only slightly disturbed by the passage of the mobile target.
Furthermore, document DE 19 803 018 also proposes a sensor operating at magnetic induction values close to Zero Gauss. However, it should be noted that the sensitive probe is set back from the magnet extension plane and consequently the plane in which the mobile target is moving, such that good measurement sensitivity cannot be obtained.